Bone fixated, articulated joint load control device

ABSTRACT

A load control device can be attached to bones on either side of an articulated joint in order to control the forces and loads experienced by the joint. The device comprises an apparatus for controlling the load on articular cartilage of a human or animal joint and includes: a first fixation assembly for attachment to a first bone; a second fixation assembly for attachment to a second bone; a link assembly coupled to the first fixation assembly by a first pivot and coupled to the second fixation assembly by a second pivot, the first and second fixation assembly thereby each being angularly displaceable relative to the link assembly. The apparatus enables a clinician to effectively control the environment of cartilage in a joint during a treatment episode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 12/628,866, filedDec. 1, 2009, which is a continuation of U.S. Ser. No. 12,425,969, filedon Apr. 17, 2009, which is a divisional of 10/675,855, filed on Sep. 25,2003, now U.S. Pat. No. 7,763,020 and claims benefit to InternationalApplication No. PCT/GB02/00844, filed Feb. 27, 2002 the entiredisclosure of which is expressly incorporated herein by reference.

BACKGROUND

The present invention relates to devices for restricting or controllingthe movement or loading levels on joints in the human or animal body.

The human or animal body uses articular cartilage to surface many of itsjoints. This tissue tolerates relatively high levels of compressionwhile having a low coefficient of friction—approximately that of wet iceon wet ice.

Bone, on which the cartilage is supported, is stiffer and stronger. Awayfrom the joints, bone normally forms in large, thick-walled tubes.However, under the cartilage at the joints, the bone forms a threedimensional mesh of so called “cancellous” bone. Cancellous bone is morecompliant than the rest of the bone structure and helps spread the loadthat the cartilage experiences, thus reducing the peak stresses on thecartilage.

Both cartilage and bone are living tissues that respond and adapt to theloads they experience. There is strong evidence that the loads thatjoint surfaces experience can be categorised into four regions or“loading zones”.

1. Under-Loading Zone.

If a joint surface remains unloaded for appreciable periods of time thecartilage tends to soften and weaken.

2. Healthy Zone.

Joint surfaces can and do last a lifetime and if they experience healthylevels of load they can be considered to effectively last indefinitely.

3. Tolerant Zone.

As with engineering materials that experience structural loads, bothbone and cartilage begin to show signs of failure at loads that arebelow their ultimate strength. Unlike engineering materials, however,cartilage and bone have some ability to repair themselves, bone more so.There are levels of loading that will cause micro-structural problemsand trigger the repair processes. The body can tolerate these loadlevels as long as it has time to recuperate.

4. Overloaded Zone.

There comes a level of load at which the skeleton will failcatastrophically. If the load level on a joint surface reaches thislevel even once then there will be severe consequences.

One of the major consequences of excessive loading is osteoarthritis.This loading could be either from a single overload in the overloadedzone or from loading within the tolerant zone too frequently.

The picture of safe joint loading is further complicated by the cascadeof events that occur during the onset of osteoarthritis. These eventsinclude the break up of the cartilage, and bone ‘sclerosis’ in which thebone becomes denser and stiffer. This means that the maximum level ofloading that can be considered healthy or tolerated falls, almostcertainly to levels below that experienced in walking and standing.

Newly implanted grafts or tissue-engineered constructs will also havelower tolerance limits while they are establishing themselves within thejoint.

In fact the treatment of osteoarthritis and other conditions is severelyhampered when a surgeon is not able to control and prescribe the levelsof joint load. Furthermore, bone healing research has shown that somemechanical stimulation can enhance the healing response and it is likelythat the optimum regime for a cartilage/bone graft or construct willinvolve different levels of load over time, eg. during a particulartreatment schedule.

There is a need for a device that will facilitate the control of load ona joint undergoing treatment or therapy, to enable use of the jointwithin the healthy loading zone, or even within the healthy and tolerantloading zones, during the treatment episode.

There is further need for a device to preferably provide such controlwhile allowing full, or relatively full mobility of a patient undergoingthe treatment.

Such devices would be desirable particularly during the early treatmentof, for example, an osteoarthritic joint. Under an appropriate treatmentregime providing controlled loading, the condition of the joint mayimprove, possibly back to full health.

In the prior art, existing load controlling regimes and devices for usein treatment or therapy of articulating joints include the following.

-   -   a) Bed-rest or isolation of a joint is possible but, as        indicated above, the long-term consequences of applying no load        or generally maintaining the joint in the underloaded zone are        not good.    -   b) Passive movement of a joint has been tried with some success.        During this treatment, movement is applied to the joint by an        external device while the joint is rested. However, this does        not give the opportunity to vary the load levels on the joint,        eg. to work the joint within the healthy zone for that joint at        any given stage of the treatment program.    -   c) Traction across a joint has long been used to counteract the        compressive loads normally experienced by the joint. This is        done either in bed or using an external fixator. Fixators exist        which not only apply traction, but also have simple hinges to        allow some joint motion.    -   d) External braces have been used to apply a bending moment        across the joint and at 90.degree. to the motion to move the        centre of pressure from one part of the joint to another.        However, since these braces are not attached directly to the        skeleton, control of the applied loads is poor.

According to one aspect, the present invention provides an apparatus forcontrolling the load on articular cartilage of a human or animal jointcomprising:

-   -   a first fixation assembly for attachment to a first bone;    -   a second fixation assembly for attachment to a second bone; and    -   a link assembly coupled to the first fixation assembly by a        first pivot and coupled to the second fixation assembly by a        second pivot,    -   the first and second fixation assembly thereby each being        angularly displaceable relative to the link assembly.

According to another aspect, the present invention provides a method ofcontrolling loading on a joint comprising the steps of:

-   -   attaching a first fixation assembly to a first bone;    -   attaching a second fixation assembly to a second bone, the        second bone being connected to the second bone by an        articulating joint;    -   coupling said first fixation assembly and said second fixation        assembly by way of a link assembly so that said first fixation        assembly and said second fixation assembly are each angularly        displaceable relative to the link assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a fixator for controlling loads onarticular cartilage according to a preferred embodiment of the presentinvention;

FIG. 2 shows a perspective view of a pair of fixators of FIG. 2 in adual sided or bilateral configuration;

FIG. 3 shows perspective views of a selection of central modulessuitable for use with the fixators of FIGS. 1 and 2;

FIG. 4 shows a perspective view of an externally powered fixator;

FIG. 5 shows perspective views of a selection of alternative fixationassemblies suitable for use in the fixators of FIGS. 1 and 2;

FIG. 6 shows a plan view of a bilateral configuration of fixators as inFIG. 2, illustrating the effects of unilateral variation in the lengthof link assembly; and

FIG. 7 shows a parallel-crosswise configuration of link assembly.

DETAILED DESCRIPTION

With reference to FIG. 1, there is shown an articulated joint loadcontrolling device or fixator 10 according to one embodiment of theinvention. The fixator 10 comprises a first fixation assembly 11, asecond fixation assembly 12 and a link assembly 13 connecting the firstand second fixation assemblies 11, 12.

The first and second fixation assemblies 11, 12 are each coupled to thelink assembly 13 by a pivot 14, 15 or other equivalent meansfacilitating angular displacement of the respective fixation assembly tothe link assembly. Throughout the present specification, use of the word“pivot” is intended to encompass all such equivalent means forfacilitating angular displacement. It will be understood that the firstand second fixation assemblies 11, 12 are therefore not only angularlydisplaceable relative to one another, but are also capable of somerelative translational movement subject to the geometric limitationsprovided by the link assembly 13.

Preferably, the axes of the pivots 14, 15 are parallel so that the firstand second fixation assemblies 11, 12 will rotate about the linkassembly in the same plane.

In an alternative embodiment, however, the pivots 14 and 15 might not beaxially parallel, in order to better follow the three-dimensionalmovement of a particular joint. In a further embodiment, one or bothpivots 14, 15 might be of the universal joint type, such that the pivotallows two degrees of rotational freedom rather than only a singledegree of rotational freedom, in order to better follow thethree-dimensional movement of, for example, a ball joint.

Each fixation assembly 11, 12 preferably comprises a faceplate 20 havingone or more slots 21 defined in the faceplate surface. Coupled to thefaceplate 20 is a clamp plate 22 which may be tightened onto thefaceplate 20 by way of screws, or other means known in the art.Preferably, the clamp plate includes corresponding slots 23. As shownmore clearly in FIG. 2, the face plate 20 and clamp plate 22 togetherprovide an anchorage for one or more bone pins 30 which can be screwedinto or otherwise fixed to a bone using known techniques. Other examplesof fixation assemblies are illustrated later in connection with FIG. 5.

In the arrangement of FIG. 1, a single load controlling fixator 10 maybe attached to an articulating joint by way of first bone pins 30screwed into one side of a first bone using the first fixation assembly11, and second bone pins 30 screwed into a corresponding side of asecond bone using the second fixation assembly 12. The first and secondbones are on either side of an articulating joint to be controlled bythe fixator.

In the arrangement of FIG. 2, two load controlling devices or fixatorsmay be used in a bilateral configuration on either side of anarticulating joint, the bone pins 30 passing right through therespective first and second bones on either side of the articulatingjoint. By applying compression in one fixator and tension in the otherfixator, it is possible to apply a bending moment to the joint so as tomove the centre of pressure within the joint in a controlled manner andso relieve the loads experienced by the areas of concern.

In further embodiments, first and second fixation assemblies 11, 12 of afixator 10 might be coupled to two or more link assemblies in series orin parallel with one another. For example, as shown in FIG. 7, a fixator16 comprises a first fixation assembly 11 and a second fixation assembly12 that are connected by a link assembly that comprises a pair of linkmembers 18, 19 in a parallel-crosswise configuration. Each link memberis pivotally anchored to both the first and second fixation assemblies11, 12 by way of face plates 17, the link members being laterallydisplaced from one another. In the embodiment shown, the link member 18and link member 19 are not only laterally displaced from one another,but also angularly displaced from one another, in a crosswise formation.This arrangement provides a controlled, limited degree of freedom ofrelative movement of the first and second fixation assemblies. Byadjusting the position of the two link members it is possible to mimicthe movement of the knee.

Referring now to FIG. 3, various arrangements of link assemblies andtheir respective functions will now be described.

In a first arrangement, labelled FIG. 3A, the link assembly 40 comprisesa rigid, fixed length member having a barrel centre section 41 and apair of lugs 44 extending from each end. Each pair of lugs 44 includes apair of coaxial apertures or hubs 42, 43 in which can rotate respectivepivot pins 14, 15 (FIG. 1). Each pair of lugs 44 define therebetween aslot 45 adapted to receive a corresponding lug 25 (see FIG. 1) of arespective fixation assembly 11 or 12. The link assembly 40 essentiallymaintains first and second pivots 14 and 15 at a fixed distance ofseparation.

In a further embodiment, the lug pairs 44 and barrel centre section 41may be screwed together for quick disassembly and re-assembly, enablingdifferent length barrel centre sections 41 to readily be used to providea link assembly 40 of an appropriate length to the joint under treatmentor therapy and to be changed during a treatment program.

In another arrangement, labelled FIG. 3C, a link assembly 50 providesfor a variable distance of separation of pivots 14, 15 in hubs 52, 53.Link assembly 50 comprises a pair of lugs 54 and a pair of lugs 55, eachpair being mounted on a central shaft 56 and being axially displaceabletherealong. A pair of tension springs 57, 58 provide a means for biasingthe distance of separation of the pivots 14, 15 towards a minimum limitof separation of the lug pairs 54, 55 to apply greater compressionforces than those normally experienced by the joint.

In another arrangement, labelled FIG. 3D, a link assembly 60 providesfor a variable distance of separation of pivots 14, 15 in hubs 62, 63.Link assembly 60 comprises a pair of lugs 64 and a pair of lugs 65, eachpair being mounted on a central shaft 66 and being axially displaceabletherealong. A compression spring 67 provides a means for biasing thedistance of separation of the pivots 14, 15 towards a maximum limit ofseparation of the lug pairs 64, 65 so as to counteract the naturalcompressive forces experienced by the joint.

It will be understood that the functions of link assembly 50 and linkassembly 60 may be combined to provide bias towards a central positionso that there is resistance against movement of the pairs of lugs from acentre position. More generally, this provides means for biasing thefirst and second pivots towards an intermediate distance of separationbetween predetermined limits of separation of the lug pairs 54, 55 or64, 65.

Although not shown in FIGS. 3C or 3D, it is also possible to provide alocking member which is axially adjustable along the length of the linkassembly to adjust the limit or limits of separation of the lug pairs.The locking member could be provided, for example, by way of ascrew-threaded collar on the central shaft 56 or 66 using techniquesthat will be understood by those skilled in the art.

In another arrangement, labelled as FIG. 3F, the provision of a meansfor controlling the distance of separation of pivots 14, 15 could be byway of a link assembly 80 that includes a pneumatic or hydrauliccylinder 81, controlled externally by a controller (not shown) connectedthereto by two feed pipes 82, 83. The pneumatic or hydraulic cylindermay also provide means for biasing the distance of separation of thefirst and second pivots to a predetermined position.

It will be understood that the functions of the pneumatic or hydrauliccylinder 81 could be alternatively provided by an electrically drivensystem.

In another arrangement, labelled FIG. 3E, the link assembly 70 (whichmay generally correspond with a links 50 or 60 having variableseparation members) may also be provided with a mechanism for varyingthe distance of separation of the pivots 14, 15 according to the angulardisplacement of the first and/or second fixation assemblies relative tothe link assembly. This would enable, for example, the separation to beincreased in the last 5° of angular displacement.

In the preferred embodiment shown, a cam surface 76, 77 is provided onthe circumferential edge of one or both of the lug pairs 74, 75including a pair of coaxial apertures or hubs 72, 73. The cam surfacebears on a corresponding bearing surface on a respective fixationassembly 11, 12 and is preferably adapted to vary the separation of thefixation assemblies as a function of the angular displacement. As anexample, for a fixator attached to a knee joint, the cam surfaces 76, 77can be arranged so that as the knee is moved to the fully extendedcondition, the fixator ensures a greater separation of the fixationassemblies 11, 12 thereby reducing pressure on the joint surfaces.

In another arrangement, the cam surfaces 76, 77 could be adapted tolimit the angular displacement of that fixation assembly.

In another arrangement, the cam surfaces may be used to provide avarying degree of resistance to angular displacement of the fixationassembly. More generally, the cam surface may be adapted to provide ameans for progressively increasing resistance to angular displacement ofthe fixation assembly relative to the link assembly as a function of theangular displacement from a reference position.

The means for limiting angular displacement could alternatively beprovided by a stepped surface on the circumferential edge of the lug ina manner which will be understood by those skilled in the art.

In conjunction with any of the link assemblies described above, a linkassembly 90 as shown in FIG. 3B may be provided with means for recordingloads applied across the link assembly. The sensor may be adapted tomonitor any one or more of the tensile load, the compression load, shearforces or bending forces applied across the link assembly. Preferablythe sensor comprises a strain gauge. Such a device makes it possible todetermine the load actually being carried by a joint. In the preferredembodiment shown, this is achieved by the installation of strain gauges92 into the barrel 93 of the link assembly 90.

Separate transducers could be added to monitor angular displacement ofthe fixation assemblies relative to the link assembly 90.

In another arrangement, as shown in FIG. 4, the angular displacement ofthe fixation assemblies relative to one another may be controlledexternally. This can be achieved by a linear actuator 100 linked to thefirst and second fixation assemblies 11, 12 by way of brackets 17, 18each extending from a respective fixation assembly in a directionorthogonal to the pivot axis.

The linear actuator 100 may be powered electrically, pneumatically orhydraulically and enables movement of a joint to be automaticallycontrolled for exercise within the healthy load zone without use of theassociated musculature.

Referring back to FIG. 2, it will be noted that when the various linkassemblies described in connection with FIG. 3 are used in the bilateralconfiguration, it is possible, by varying the length of the linkassembly 13 independently on either side of the joint, to alter theposition of the centre of pressure in the joint. This can beparticularly useful in the treatment of knees. An example of the effectsof this is illustrated in FIG. 6. In the figure, a bilateralconfiguration of fixation assemblies is shown similar to that of FIG. 2,viewed from above (ie. generally perpendicular to the pivot axes). Inthis example, fixation assemblies 11 a, 12 a and 111 b, 12 b arerespectively connected by link assemblies 13 a, 13 b. A unilateraladjustment of the length of link assembly 13 b results in a relativeangular displacement of the bones 4, 5 that varies with articulation ofthe bones about the axis of the joint, thereby imposing an angulation onthe joint. This will tend to have the effect of reducing the loadexperienced at the joint surface on the side nearest to the lengthenedsegment.

With reference to FIG. 5, alternative arrangements of fixationassemblies which themselves facilitate further angular degrees offreedom of the fixator are now described. FIG. 5A shows a fixationassembly 110 have a lug 25 for attachment to the various possible linkassemblies 13, 40, 50, 60, 70, 80, 90. The body of the fixation assemblyincludes a plurality of apertures 111 a, 111 b, 111 c each adapted toreceive a bone pin 30. In this arrangement, however, each aperture isdefined in a corresponding rotatable collar 112 a, 112 b, 112 c suchthat the angles of the bone pins 30 may be varied on and about thecentral longitudinal axis of the fixation assembly 110.

The fixation assembly 120 as shown in FIG. 5 b is similar to that ofFIG. 5 a, except that in this case the apertures 121 a, 121 b, 121 c arelaterally offset from the central longitudinal axis of the fixationassembly. Each aperture is again defined in a corresponding rotatablecollar 122 a, 122 b, 122 c so that the angles of the bone pins 30 may bevaried about a longitudinal axis that is laterally displaced from thecentral longitudinal axis of the fixation assembly 120.

The fixation assembly 130 as shown in FIG. 5 c provides a further degreeof freedom. In this arrangement, bone pins 30 are located in slots 131formed between a first clamp plate 132 and a second clamp plate 133. Theclamp plates 132, 133 are rotatable about a first axis (transverse to alongitudinal axis of the fixation assembly 130) on pivot 134, and abouta second axis (preferably a longitudinal axis of the fixation assembly130) on pivot 135. Taken together with the pivot through aperture 136,this provides a full three rotational degrees of freedom of the bonerelative to a link assembly.

The fixator device embodiments as generally described above thereforeprovide a means for applying and/or limiting tension, compression,torsion, bending and shear forces to an articulated joint in acontrolled manner and provide for some or all of the following treatmentregimes either in isolation or in any combination. Also it will bepossible to change the regime or combination of regimes easily andwithout the need for a sterile environment or anaesthesia. It is notedthat joints of the skeleton naturally experience compression and thefixators of the present invention can provide amelioration of this byapplying tension.

1. Continuous Traction.

The level of tension can be varied according to the length of the linkassembly used and this can be further varied according to the biasstrengths applied by the springs 57, 58, 67.

2. Partial or Full Support.

Some or all of the compression that would otherwise be carried by thejoint may be taken by the device, and this can be a function of theangle of support by control of spring strength and angle of fixation tothe bones.

3. Application of a Bending Moment, Torsion or Shear Force.

Application of these loads to the joint allows the clinician to move thecentre of pressure within the joint to regions that are healthy.

4. Application of an Externally Powered Loading Regime.

This can occur normally while the subject is at rest at set angles, loadlevels, loading and unloading rates and frequencies using the poweredembodiments described above. Providing a portable power supply will,however, allow the patient to continue to move freely.

5. Allowing the Joint Load to be Gradually Increased.

This may be desirable at the end of a treatment episode. This can bedone either by applying additional compression or a bending moment,shear force or torsion in the opposite direction from that describedabove.

6. Load Measurement.

The device as described in connection with FIG. 3B allows the clinicianto detect and record the loads experienced across a joint, and also theload applied across the joint by the device.

The motive forces applied across the joints may be from normalphysiological loads of the musculoskeletal system or from an externallyapplied source such as described with reference to FIG. 4.

It will be noted that the preferred design of the devices describedabove enable the link assemblies and fixation assemblies to be readilydisconnected from the bone pins 30 in order to replace or adjust thedevices during a treatment schedule. Still further, in the preferreddesigns, the link assemblies may be adjusted in situ. Preferably, thedevices will be able to be removed in their entirety within anoutpatient clinic.

The articulated joint controlling devices of the present invention canbe used in the treatment not only of rheumatoid arthritis, but for thetreatment of many other conditions such as articular fractures, andfollowing surgical procedures such as osteochondral transfers and jointsurface replacement with cartilage graft.

Other embodiments are within the scope of the appended claims.

1. A device for controlling load on a knee joint, the device comprising:first and second fixation assemblies configured for attachment to a sideof a knee joint; a link assembly connectable to the first and secondfixation assemblies at first and second pivots respectively, wherein thelink assembly is configured to provide a variable distance of separationof the first and second pivots; wherein the link assembly comprises aspring; and wherein the spring is arranged in the link assembly to biasthe first and second pivots away from one another.
 2. The device ofclaim 1, wherein the first and second pivots comprise articulatingsurfaces.
 3. The device of claim 1, wherein the link assembly isconfigured to be positioned entirely outside of the articular surfacesof the knee joint.
 4. The device of claim 1, further comprising a firstfastener configured to extend through the first fixation assembly andinto the femur; and a second fastener configured to extend through thesecond fixation assembly and into the tibia.
 5. The device of claim 1,wherein the spring comprises a coil spring.
 6. The device of claim 1,wherein the link assembly is configured to act in compression tocounteract the natural compressive forces experienced by the knee joint.7. The device of claim 1, wherein the link assembly comprises means toapply a force in a direction of distraction of the knee joint.
 8. Thedevice of claim 1, wherein the link assembly comprises a central shaft.9. The device of claim 8, wherein the link assembly comprises a hubmounted to the central shaft and configured to contact an end of thespring.
 10. The device of claim 8, wherein the central shaft extendsthrough the spring.
 11. A device for controlling load on a joint, thedevice comprising: first and second fixation assemblies configured forattachment to a side of a joint; a link assembly connectable to thefirst and second fixation assemblies at first and second pivotsrespectively, wherein the link assembly is configured to provide avariable distance of separation of the first and second pivots; whereinthe link assembly comprises a spring; and wherein the spring is arrangedin the link assembly to bias the first and second pivots away from oneanother.
 12. The device of claim 11, wherein the first and second pivotscomprise articulating surfaces.
 13. The device of claim 11, wherein thelink assembly is configured to be positioned entirely outside of thearticular surfaces of the joint.
 14. The device of claim 11, furthercomprising a first fastener configured to extend through the firstfixation assembly and into a first bone of the joint; and a secondfastener configured to extend through the second fixation assembly andinto a second bone of the joint.
 15. The device of claim 11, wherein thespring comprises a coil spring.
 16. The device of claim 11, wherein thelink assembly is configured to act in compression to counteract thenatural compressive forces normally experienced by the joint.
 17. Thedevice of claim 11, wherein the link assembly comprises means to apply aforce in a direction of distraction of the joint.
 18. The device ofclaim 11, wherein the link assembly comprises a central shaft.
 19. Thedevice of claim 18, wherein the link assembly comprises a hub mounted tothe central shaft and configured to contact an end of the spring. 20.The device of claim 18, wherein the central shaft extends through thespring.
 21. A device for controlling load on a joint, the devicecomprising: first and second fixation assemblies configured forattachment to a side of a joint; a link assembly connectable to thefirst and second fixation assemblies at first and second pivot meansrespectively, wherein the link assembly is configured to provide avariable distance of separation of the first and second pivot means;wherein the link assembly comprises a means for biasing; and wherein themeans for biasing is arranged in the link assembly to bias the first andsecond pivot means away from one another.
 22. The device of claim 21,wherein the first and second pivot means comprise articulating surfaces.23. The device of claim 21, wherein the link assembly is configured tobe positioned entirely outside of the articular surfaces of the joint.24. The device of claim 21, further comprising a first fastener meansconfigured to extend through the first fixation assembly and into thefemur; and a second fastener means configured to extend through thesecond fixation assembly and into the tibia.
 25. The device of claim 21,wherein the means for biasing comprises a coil spring.
 26. The device ofclaim 21, wherein the link assembly is configured to act in compressionto counteract the natural compressive forces experienced by the joint.27. The device of claim 21, wherein the link assembly comprises means toapply a force in a direction of distraction of the joint.
 28. The deviceof claim 21, wherein the link assembly comprises a central shaft. 29.The device of claim 28, wherein the link assembly comprises a hubmounted to the central shaft and configured to contact an end of themeans for biasing.
 30. The device of claim 28, wherein the central shaftextends through the means for biasing.